For many years, we used hand cut Copper foil covered with a thin layer of Bees wax on the backside

to frame our glass pieces for soldering.

Here is a photo taken from an exhibition at the Queen Museum in NY , which shows one of the old tools used by Tiffany Studios.
 

Today we use a modern cutting machine, the result is the same.

The photos show that  the glass fits perfectly on the new lamp mold.